Klahn first played tennis at the age of 11. He celebrated his first successes at the ITA Division I Regional Championships in 2009 and at Stanford University , where he won the NCAA Singles Championships for his team in 2010 . He played his first game on the ATP World Tour , thanks to a wildcard , at the 2010 US Open , where he failed in the first round against his compatriot Sam Querrey . He finished the year at position 798 in the tennis world rankings . In February 2011, he played his second game at this level and again lost his opening game against Gaël Monfils at the tournament in San José . He had his first success at the highest level at the US Open 2012 , where he defeated the Austrian Jürgen Melzer 4: 6, 6: 3, 7: 5, 5: 7 and 6: 4 in a five-sentence thriller . In the second round he failed clearly in three sets against the French Richard Gasquet . Since then, Klahn has been on the ATP World Tour again and again, but without any major success. On the ATP Challenger Tour he reached his first final in Winnetka in July 2013 , which he lost to Jack Sock in two sets. That same month he was defeated in another final in Binghamton against Alex Kuznetsov in three sets. In August 2013 he finally won his first individual tournament on the Challenger Tour when he beat Daniel Evans in three sets in Aptos .
After he last played matches in early 2015, he had to undergo an operation on his back, which previously caused him problems. After a 21-month hiatus, he returned to the tour in late 2016 when he received a wildcard to qualify for the Challenger in Champaign . There he advanced to the quarter-finals. In 2017 he played mainly on the Challenger Tour and won the doubles competition in Gatineau .
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United States
Daniel Evans
Yeongwol
Taro Daniel
Yang Tsung-hua
Adelaide
Gatineau
Ugo Humbert
Michael Venus
Sanchai Ratiwatana Sonchat Ratiwatana
Hans Hach Verdugo Vincent Millot
Guillermo Durán Máximo González
Darian King Peter Polansky